London attack: Officer stabbed as victims lay bloodied in the street

Witnesses watched a terrorist plunge a blade into the neck of a London police officer as victims he mowed down with a car lay in pools of blood on the street.

Five people are dead and at least 40 have been injured by a man who ran down crowds of people on the Westminster bridge before hitting the gates of London's Parliament House and leaping out with two knives on Thursday.

Hundreds of terrified people surged through a cobble square opposite the Big Ben as the man brutally attacked an officer who was guarding the perimeter.

"A guy came past my right shoulder with a big knife and just started plunging it into the policeman," a witness told the Telegraph.

A person is taken away on a stretcher outside Westminster Palace. (Getty) ()

Kevin Schofield said the man was making his way toward another officer when police opened fire.

"He was walking towards a second policeman with his arm outstretched, with what looked like a knife in his hand, having already assaulted another policeman."

People scattered as up to six shots rang out.

Quentin Letts said two plain clothed officers tried to warn the knifeman to stop but were forced to shoot when he continued moving toward them.

"As this attacker was running towards the entrance two plain-clothed guys with guns shouted at him what sounded like a warning, he ignored it and they shot two or three times and he fell," he told the BBC.

The car believed to have been driven into a crowd of people on Westminster Bridge in London. (Twitter) ()

The man left a trail of devastation in his wake before he was shot. Bodies were strewn along the Westminster Bridge, where he used his vehicle as a weapon – striking down dozens.

"I just saw people being hit by a car at speed, it was harrowing. I just sort of went into shock mode," witness Rob Lyon said.

"I was numb it felt like I was in the middle of a Hollywood movie, you see these things but suddenly I was in the middle of it," he added.

Another man described the scene as traumatised victims lay bloodied on the street and emergency services tried to work out who most urgently needed treatment.

"There was a woman at the top of the steps, her head was just pulsing with blood and bleeding out and there was another woman who flipped over the side of the bridge. Someone landed in the river somewhere," a witness told The Sun.

As the chaos subsided, witnesses said they started to notice which of the victims were too seriously injured to get up.

"I saw a trainer lying in the road and when I looked more closely I saw that there were a couple of bodies the other side of the road," a man told the Press Association.

"And when I looked over the side there was another body lying in the water with blood all around it."

Five people are confirmed dead, including the man behind the attack.

Police believe he acted alone but have locked down much of the surrounding city as investigations continue.